The present invention relates to a recording and reproducing apparatus of the type using an optical disk such as a magneto-optical disk and, more particularly, to a focus servo circuit incorporated in such an apparatus for focusing a laser beam onto a recording medium surface or a recording film surface of a magneto-optical disk accurately and rapidly without an error.
A recording and reproducing apparatus of the type described records and reproduces information in and out of a magneto-optical disk by striking a laser beam onto the disk. This kind of apparatus has a tracking servo circuit for accurately positioning the laser beam on a desired track of the disk, and a focus servo circuit for focusing the laser beam onto a recording film surface of the disk. The prerequisite with the focus control is that the laser beam be focused onto the surface of the recording film which is positioned 1 millimeter to 2 millimeters deep in the disk as measured from the surface of the disk, and not onto the surface of the disk. Since the surface of the recording film generally has greater reflectivity than that of the disk, the focus servo control is so effected as to focus the laser beam onto the recording film on the basis of such a difference in reflectivity. However, when use is made of a disk having a recording film whose surface is merely a little different in reflectivity from the surface of the disk, the focus servo is apt to focus the laser beam onto the disk surface.
To better understand the present invention, a brief reference will be made to a conventional recording and reproducing apparatus, shown in FIG. 1. As shown, the conventional apparatus has a magneto-optical disk 1, an optical head 2, a laser diode (LD) driver 3, a magnetic field generating unit 4, a radio frequency amplifier (RF AMP) 5 for amplifying a reproduced RF signal, a focus servo circuit 6, and a tracking servo circuit 7. On receiving a data signal, the LD driver 3 modulates a laser beam by the data signal in a predetermined manner to thereby record the data signal in the recording film of the disk.
The head 2 has a laser diode 8, a collimator lens 9, a beam shaping prism 10, polarizing beam splitters 11 and 12, and an objective lens 13. A laser beam issuing from the laser diode 8 is incident to the disk 1. The light from the beam splitter 12 is propagated through a 1/4 wavelength plate 14 and a lens 15 to a detector 16 which is assigned to the reproduced signal, whereby the reproduced signal is picked up. The light from the other beam splitter 11 is propagated through a lens 17 and a beam splitter 18 to a detector 19 which is assigned to tracking error detection, whereby a signal for tracking error detection is picked up. The other part of the light from the beam splitter 18 is incident to a detector 21 for focus error detection via a knife edge 20 so as to pick up a signal for focus error detection. The outputs of the tracking servo circuit 7 and focus servo circuit 6 are applied to a lens actuator 22 to control the position of the objective lens 13.
FIG. 2 shows a prior art construction of the focus servo circuit 6. As shown, a signal S.sub.1 photoelectrically converted by the detector 21 is transferred to a differential amplifier (DIF. AMP) 24 via an inverting amplifier 23. The differential amplifier 24 produces a difference signal representative of a difference between the amplified signal S.sub.1 and a signal S.sub.2. An error amplifier (ERR. AMP) 25 performs servo operations such that the error signal from the differential amplifier 25 becomes zero, i.e., such that the input has the ground level. The output of the error amplifier 25 is corrected in phase by a lead-lag filter 26 and then fed to a drive amplifier (DRIVE AMP) 29 via a switch 27 and a mix amplifier (MIX AMP) 28. The drive amplifier 29 effects voltage-to-current conversion to drive a coil which is included in the actuator 22, FIG. 1.
At the beginning of servo pull-in, a CPU 30 opens the switch 27 to open the servo loop (servo OFF) and then delivers data for moving the objective lens 13 to a digital-to-analog converter (D/A CONV.) 31. These data are selected such that the objective lens 13 sequentially approaches the surface of the disk 1 away from a position which is sufficiently spaced apart from the latter. The D/A converter 31 sequentially converts the incoming data to an analog signal to thereby drive the previously mentioned coil via the mix amplifier 28 and drive amplifier 29. On the other hand, a sum amplifier (SUM AMP) 34 produces a sum signal which is representative of the sum of the signals S.sub.1 and S.sub.2. The sum signal and the previously mentioned difference signal are applied to a sum level comparator 32 and a pull in comparator 33. While the objective lens 13 is in movement, the CPU 30 constantly monitors the outputs of the comparators 32 and 33. As soon as the sum signal and the difference signal exceeds respectively predetermined threshold values Vths and Vthd, the CPU 30 closes the switch 27 to end the servo pull-in operation.
The above-described operation will be described with reference to FIGS. 3(a), 3(b) and 3(c) which show respectively the sum signal of signals S.sub.1 and S.sub.2, the difference signal, and the position of the lens 13 relative to the disk 1. In FIG. 3(c), L.sub.S is representative of a distance where the focus of the objective lens 13 is coincident with the surface of the disk, while L.sub.R is representative of a distance where the focus of the lens 13 is coincident with the surface of the recording film of the disk. In a magneto-optical disk, the recording film surface is located at a position deeper than the disk surface. FIG. 3(d) indicates the drive current which is produced by the drive amplifier 29, FIG. 2.
First, a current I, FIG. 3(d), which sequentially increases from I'.sub.p is applied to the lens actuator to move the objective lens 13 little by little from a position P' sufficiently spaced apart from the disk surface toward the disk surface. The current I corresponds to the previously mentioned data which is transferred from the CPU 30 to the D/A converter 31. In general, the objective lens 13 is moved in such a manner away from the above-mentioned position. The reason is that the surface of the recording film which generally deviates over a range of about .+-.100 microns will be surely brought to the focal depth despite that the detectable range of the focus detector of the optical reproducing, apparatus is as narrow as about .+-.20 microns.
Ideally, it is possible that the objective lens may be once brought close to the disk surface and then sequentially moved away from the latter. However, this scheme is generally not used since the lens moved beyond the focusing point might hit against the disk surface.
As the objective lens 13 approaches the disk, the servo error signal (difference signal) and the sum signal change as shown in FIGS. 3(a) and 3(b), respectively. When the sum signal and the difference signal exceed respectively the thresholds values Vths and Vthd, it is determined that the focusing point has entered the pull-in range. As a result, the servo loop is closed.
Assume that the distance between the objective 13 lens and the surface of the disk which faces the lens 13 is generically referred to as L, and that the specific distance at which the light beam is focused onto the disk surface that faces the lens is L.sub.S. Then, the distances L and L.sub.S are related as shown in FIG. 3(c). At this instant, the disk 1 and the objective lens 13 are related as shown in FIGS. 4(a) and 4(b). In the figures, the recording film is labeled la and positioned deeper than the disk surface.
The conventional method described above has the following problem. Assume that the reflectivity of the disk surface and that of the recording surface are close to each other, as indicated by dashed lines in FIG. 3. Then, since the conventional method allows a condition L=L.sub.S to hold during the focus servo pull-in operation, the sum signals Vss and Vfs and the difference signals Vsd and Vfd appearing at two locations of the recording film surface are close to each other. As a result, the pull-in condition is satisfied when the light beam is focused on the disk surface, closing the servo loop. More specifically when the reflectivity of the surface of the recording film is close to that of the disk surface, it is likely that the servo loop is closed when the light beam is focused on the disk surface. This disturbs the performance of an optical recording/reproducing apparatus in normal recording or reproducing operation.